Stromal-immune cell crosstalk in the developing brain borders

Central nervous system (CNS) border tissues including the meninges serve as key immune interfaces between the brain and the periphery. The three meningeal layers – the pia mater, the arachnoid mater, and the dura mater - each host a unique landscape of immune lineage cells that continuously shape healthy adult brain function and provide a firewall against invasion of pathogens into the central nervous system. However, much less is known about how the meningeal immune compartment develops in early life or how meningeal immune cells may interact with or protect the brain during critical developmental windows. Emerging evidence also suggests that meningeal stromal cells critically shape the adult meningeal immune landscape. How meningeal stromal cells contribute to establishment of this compartment in the early life window is an outstanding question. To better understand the how the meningeal immune compartment is established, we profiled changes in the composition of meningeal immune cells from early postnatal life through adulthood. Unexpectedly, we uncovered a role for the meninges as a niche for B lymphopoiesis in the early postnatal window. B cells appear in the dura mater and other meningeal compartments as a wave spanning the first month of life in mice. Meningeal B cells undergo lymphopoiesis locally but in concert with other waves of extramedullary lymphopoiesis across the body that fuel generation of the B2 B cell compartment. Developing B cells in the dura are seeded by a common pool of hematopoietic progenitors as are B cells in other early life organs but diverge in the perinatal window and establish local hematopoietic foci in microanatomical niches near the dural venous sinuses. To elucidate the niches that support meningeal lymphopoiesis and their relationship to bone marrow niches, we profiled stromal and endothelial cells from the early life dura mater and bone marrow. This data revealed that dura and bone marrow niches are fundamentally distinct in composition. Indeed, the early life dura mater lacks canonical mesenchymal stromal cell populations known to regulate B cell development and instead comprises a heterogenous landscape of fibroblast-like cells (FLCs) enriched for the transcription factor Foxd1. Lymphopoietic foci associate with a unique network of dural sinus-associated (peri-sinus) FLCs that express the pro-hematopoietic chemokine Cxcl12, and ablation of Cxcl12 from Foxd1-expressing FLCs profoundly impairs local B lymphopoiesis. These data suggest that CNS border tissues contribute to generation of the postnatal B cell compartment and provide a model for how extramedullary stromal niches may regulate lymphopoiesis across the body in early life. Separately, we have undertaken broader efforts to define functional roles for CNS immune cells in health and disease. One of these includes a collaborative effort to develop FEAST (Flow cytometric Engulfment Assay for Specific Target proteins) - a robust, in vivo assay to quantify engulfment of neuronal and myelin substrates by brain and brain border macrophages. This approach will be useful for studying the role of CNS phagocytes in tissue development, infection, and other neurological diseases across the lifespan.